1. Field of the Invention
This invention relates to a method and apparatus for passing optical fibers through tubular products, and more particularly to a method and apparatus for making optical fiber core wires, optical fiber cords and/or optical fiber cables comprising optical fibers passed through protective tubes or sheaths.
For the purpose of this invention, optical fibers are defined as element fibers comprising a core and a cladding layer, such element fibers as are coated with syntetic resins, metals and ceramics and their variations comprising a single fiber, multiple fibers and stranded fibers. Tubular products are such metal tubes as those of steel and aluminum, and such nonmetal tubes as those of plastic.
2. Description of the Prior Art
Recently optical fiber cables have come to be used widely for communications services. And many of them are metal-coated to make up for limited strength of optical fibers.
Optical fibers passed through metal and other tubes have been made by a method that combines tape forming and welding (such as that disclosed in Japanese Provisional Patent Publication No. 46869 of 1985) and a method that passes an optical fiber through a tube (such as that disclosed in Japanese Provisional Patent Publication No. 25606 of 1983).
In the former method, an optical fiber is passed through a metal tube while a metal tape is being formed into a tubular shape and both edges of the tape are being welded together. But there is a shortcoming that the optical fiber is liable to degenerate under the influence of welding heat when it passes the welding point. Also, an optical fiber is difficult to pass through tubes whose diameter is as small as 2 mm or under.
In the latter method, an aluminum tube is made with a steel wire passed therethrough. After the tube is subjected to a diameter-reducing process, the steel wire inside the tube is replaced with an optical fiber. This method requires intricate processes. Besides, the force with which the steel wire is pulled out for replacement should not exceed the strength of the optical fiber in order to avoid the risk of fiber breaking. Accordingly, optical fiber cable having a length of 200 m or more have been difficult to make.